Technical Field
The invention relates generally to precision measurement instruments, and particularly to absolute position encoders that may be utilized in handheld instruments such as calipers.
Description of the Related Art
Various position transducers are available, such as optical, capacitive, and inductive transducers. These transducers may use a transmitter and a receiver in a read head to measure its movement relative to a scale. Some types of transducers are sensitive to contamination, such that using them in manufacturing or shop environments is impractical. In contrast, inductive sensors may be immune to contamination by particles, oil, water, and other fluids. U.S. Pat. No. 6,011,389 (the '389 patent), which is hereby incorporated herein by reference in its entirety, describes an induced current position transducer usable in high accuracy applications. U.S. Pat. Nos. 5,973,494 (the '494 patent) and 6,002,250 (the '250 patent), which are each hereby incorporated herein by reference in their entireties, describe incremental position inductive calipers and linear scales, including signal generating and processing circuits. U.S. Pat. Nos. 5,886,519, 5,841,274, and 5,894,678, which are each hereby incorporated herein by reference in their entireties, describe absolute position inductive calipers and electronic tape measures using the induced current transducer. As described in these patents, the induced current transducer is readily manufactured using known printed circuit board technology.
Different implementations of the induced current transducer (and other types of transducers) may be implemented as either incremental or absolute position encoders. In general, incremental position encoders utilize a scale that allows the displacement of a read head relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. However, in certain applications, such as those where encoders are used in low power consumption devices, it is more desirable to use absolute position encoders. Absolute position encoders provide a unique output signal, or combination of signals, at each position (of a read head) along a scale. They do not require continuous accumulation of incremental displacements in order to identify a position. Thus, absolute position encoders allow various power conservation schemes.
In addition to the '519, '274, and '678 patents described above for the absolute induced current transducer, U.S. Pat. Nos. 3,882,482, 5,965,879, 5,279,044, 5,237,391, 5,442,166, 4,964,727, 4,414,754, 4,109,389, 5,773,820, 5,010,655, 6,335,618, and U.S. Patent Publication No. 2015/0362336 (Ser. No. 14/303,266, filed Jun. 12, 2014; 2014 August) also disclose various encoder configurations and/or signal processing techniques relevant to absolute encoders, and are each hereby incorporated herein by reference in their entirety. However, many of these disclosed systems fail to teach configurations which are robust against measurement errors due to an unintended gap variation introduced between the read head and the scale or an unintended roll, pitch and/or yaw of the read head relative to the axis of the scale. For example, as shown in FIG. 1, while the read head 164 and the scale 102 are configured to maintain a certain designed (intended) gap therebetween, uniformly along the length of the scale 102, to permit sliding movement of the read head relative to the scale for proper function of the caliper 100, the actual gap may be greater or smaller than the designed gap and may be non-uniform along the axis of the scale due to manufacturing imperfection, for example. As further shown in FIG. 1, while the axis of the read head 164 is designed to lie in parallel to the axis of the scale 102, the actual axis of the read head may be rotated around the X-, Y- and/or Z-axes to have roll, pitch and/or yaw due to manufacturing imperfection, for example. Even minor deviations in terms of a gap, roll, pitch and/or yaw may lead to measurement errors not desirable in high-precision absolute position encoders such as calipers. Improved configurations of absolute encoders that are robust against these types of measurement errors would be desirable.